July 30 (Reuters) - U.S. refiner Valero Energy Corp reported a better-than-expected quarterly profit as its refining margins rose due to a steep fall in crude oil prices.

Gasoline crack spreads - the difference between crude oil and gasoline prices - have risen due to a 50 percent slump in crude prices since June 2014.

Valero's refining margin rose to $13.71 per barrel in the second quarter from $9.84 per barrel a year earlier.

The company's refining volumes rose 3 percent to an average of 2.8 million barrels per day (bpd), mainly due to lower maintenance activity.

Valero said on Thursday that it was on track to start operations at two light crude processing units at the Corpus Christi and Houston refineries in the first quarter of 2016.

Net income from continuing operations attributable to Valero's stockholders more than doubled to $1.35 billion, or $2.66 per share, in the quarter ended June 30 from $651 million, or $1.22 per share, a year earlier.

Analysts on average had expected earnings of $2.42 per share, according to Thomson Reuters I/B/E/S.

Operating revenue fell 28 percent to $25.12 billion.

The San Antonio-based company's shares were up 1.3 percent at $66.90 in premarket trading on Thursday. Up to Wednesday's close, the stock had risen about 33 percent this year. (Reporting by Amrutha Gayathri in Bengaluru; Editing by Kirti Pandey)